Datasheet
36
8011N–AVR–01/10
ATmega164P/324P/644P
6.6 Calibrated Internal RC Oscillator
By default, the Internal RC Oscillator provides an approximate 8 MHz clock. Though voltage and
temperature dependent, this clock can be very accurately calibrated by the the user. See Table
25-4 on page 330 and ”Internal Oscillator Speed” on page 356 and page 380 for more details.
The device is shipped with the CKDIV8 Fuse programmed. See ”System Clock Prescaler” on
page 38 for more details.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 6-10. If selected, it will operate with no external components. During reset, hardware loads
the pre-programmed calibration value into the OSCCAL Register and thereby automatically cal-
ibrates the RC Oscillator. The accuracy of this calibration is shown as Factory calibration in
Table 25-4 on page 330.
By changing the OSCCAL register from SW, see ”OSCCAL – Oscillator Calibration Register” on
page 40, it is possible to get a higher calibration accuracy than by using the factory calibration.
The accuracy of this calibration is shown as User calibration in Table 25-4 on page 330.
When this Oscillator is used as the chip clock, the Watchdog Oscillator will still be used for the
Watchdog Timer and for the Reset Time-out. For more information on the pre-programmed cali-
bration value, see the section ”Calibration Byte” on page 296.
Notes: 1. The device is shipped with this option selected.
2. If 8 MHz frequency exceeds the specification of the device (depends on V
CC
), the CKDIV8
Fuse can be programmed in order to divide the internal frequency by 8.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 6-11 on page 36.
Note: 1. The device is shipped with this option selected.
Table 6-10. Internal Calibrated RC Oscillator Operating Modes
Frequency Range
(2)
(MHz) CKSEL3..0
7.3 - 8.1 0010
(1)
Table 6-11. Start-up times for the Internal Calibrated RC Oscillator clock selection
Power Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (V
CC
= 5.0V) SUT1..0
BOD enabled 6 CK 14CK 00
Fast rising power 6 CK 14CK + 4.1 ms 01
Slowly rising power 6 CK 14CK + 65 ms 10
(1)
Reserved 11